1. Field of the Invention
The present invention relates generally to solar energy conversion systems and more particularly systems employing both thermionic and photoelectric mechanisms for generating current.
2. Description of the Prior Art
From the early experiment of Becquerel, Guthrie, Edison and others, it was known that by heating a metal, it is possible to "evaporate" ions or electrons from the metal. This electron or ion emission is related to the temperature of the emitting metal. It has been found that at lower temperatures the thermionic emission from metals is predominantly positive but that at much higher temperatures the negative or electron emission rapidly surpasses the positive and becomes all-important. It is thought that the initial positive emission from metals is largely due to impurities and the positive emission is seen to decrease as a function of time even where the temperature of the metal is held such that predominantly positive emission would be expected.
It has been observed that some metals emit electrons much more copiously than others. A notable example is thorium, an observed film of which on tungsten gives very copious electron emissions at high temperatures. This high temperature electron emission generally follows the Richardson-Dushman equation: EQU I.sub.s =AT.sup.2 e.sup.-B/T
i.sub.s is the thermionic current density. A is a proportionality constant which for thoriated tungsten is about 3. T is the absolute temperature while e is the base of natural logarithms. B is a constant equal to the work function of the metal divided by the Boltzmann constant. For thoriated tungsten the valve of B is approximately 30500. Thus, for all practical purposes, the generation of usable levels of current occurs only when the emitting metal has a temperature somewhere above 1500.degree. K.
The electron emission from a metal can also be achieved photoelectrically. As first observed by Hertz, first experimentally quantified by Lenard, and satisfactorily explained by Einstein, electrons in the surface of a metal receive sufficient energy to escape from the metal when an electromagnetic wave such as light is incident upon the metal surface. The emission of the electrons is not dependent upon the emitting metal's temperature but is rather dependent upon the presence of electromagnetic radiation having a frequency greater than that value found from the quotient of the work function of that metal divided by Planck's constant. This minimum frequency may be translated as a maximum wave length of light which falling on a metal will cause electron emission. In the case of thoriated tungsten the maximum wave length is about 4700 Angstroms. Thus, for thoriated tunsten, photoelectric emission could be expected only from light in the deep-blue, violet, and ultraviolet areas of the spectrum. Further, it is observed that as the wave length of the impinging light moves increasingly toward the ultraviolet, the electrons emitted from the metal are more energetic while the current generated for any given light intensity remains unchanged with respect to variations in color. On the other hand, the photoelectric emission current is directly related to the intensity of the light incident upon the photoelectric target.
It is an object of the present invention to use both thermionic and photoelectric emission from a target so as to enhance the current generating capabilities of a physical system which might utilize either effect alone. It is a further object of this invention to physically construct a target and its surrounding environment so as to maximize both the photoelectric and thermionic emission effects.
It will be appreciated that with respect to solar powered energy sources employing combined thermionic and photoelectric effects, an increased efficiency in current production could be achieved if a greater proportion of the spectrum of light incident upon the target had a frequency greater than that minimum frequency necessary to cause photoelectric emission. Thus, it is another object of the present invention to design a solar-powered thermionic-photoelectric current generator which includes an appropriate light amplification means so as to shift a portion of the incident light spectrum toward the ultraviolet region thereby causing increased photoelectric emission current. This increase in photoelectric emission current is recognizably achieved only at some loss in the thermionic emission current. It is believed that an overall increase in current will be realized based on the greater efficiency of the photoelectric effect over that of the thermionic effect.
A problem common to both the photoelectric and thermionic production of current is the existence of a space charge which may be viewed as a gaseous sea of electrons concentrated just above the surface of the emitting metal which acts as a retarding potential to significantly decrease if not stop entirely the current which would ordinarily be produced. Various mechanical arrangements principally of shorting screens or grids have been proposed which would materially reduce the space charge thereby enhancing the current. None of these mechanical arrangements of screens or grids has performed satisfactorily. It is therefore an object of the present invention to enhance the current production by thermionic and photoelectric effects by so forming the target that the space charge of electrons can be continuously swept away from the target by the radiation pressure of the light incident upon the target at very large angles of incidence.
Generally, electromagnetic radiation possesses momentum and therefore exerts a pressure on objects on which it impinges. The effect of this pressure is most conspicuous in the case of a comet near the sun where the radiation pressure from the sun forces the lighter constituents of the comet away from the sun. While the radiation pressure from the sun on the surface of the earth is very small (about 10.sup.-9 newtons/meter.sup.2), when the sunlight incident upon a diverse area is collected and concentrated into a very narrow beam, this radiation pressure increases in direct proportion to the reduction in area. Even with the substantial reduction in area which is intended by the present invention, the radiation pressure remains very small as measured in absolute terms. However, when it is considered that this very small radiation pressure is going to act on the extremely small mass of the electrons forming the space charge adjacent the target, it will be appreciated that a substantial acceleration will be experienced by the electrons which will in turn move the electrons to a region of the current generator where they will no longer inhibit the thermionic-photoelectric current generation but rather contribute to it.